1. Technical Field
The present invention relates to a turbo compound engine that is capable of recovering the energy of exhaust gas and utilizing the recovered energy as rotative power for a drive shaft such as a crank shaft or the like. In particular, it concerns a turbo compound engine that is capable of developing an engine brake force that is at least equal to that of a non-supercharged engine of equal power output.
2. Background Art
Generally, supercharged engines feature, in comparison with non-supercharged engines of larger displacements: (1) better fuel economy, (2) output performance that is at least equal to non-supercharged engines, and (3) lighter weight and compactness.
These advantages of supercharged engines are further augmented in highly supercharged engines and in turbo compound engines, and this invention is especially concerned with the latter.
In the turbo compound engine, the energy of exhaust gas from the engine is reocvered by the turbocharger as its supercharging work, and the remaining energy of the exhaust gas from the turbocharger is recovered by the power turbine as its adiabatic expansion work. Although this construction brings about a general improvement in the power output performance, fuel economy, and gain of the engine, the expansion ratios of the turbocharger and the power turbine must be increased in order to raise its supercharging pressure so that the turbo compound engine's overall performance can be further improved.
However, it remains a problem to secure an engine brake (exhaust brake) to counterbalance the increased power output of the engine. This may be seen in the relationships existing among engine revolution speed Ne, engine output power Pme, and engine brake force Pmf, shown in FIG. 6 of the accompanying drawings for a highly turbocharged engine in comparison with a non-supercharged engine. In this figure, the solid line indicates the output power performance, and the dotted line represents engine brake force. Specifically, with regard to the relative braking force (i.e., engine revolution speed/engine output power) at the 100% rated revolution speed N.sub.100, B.sub.N /S.sub.N&gt;B.sub.T /S.sub.T, where B.sub.N stands for the engine brake force of a non-supercharged engine, S.sub.N for the engine output power of the non-supercharged engine, B.sub.T for the engine brake force of a highly supercharged engine, and S.sub.T for the engine output power of the highly supercharged engine. This means that the relative engine brake force suffers a decrease when the supercharging is intensified, so that a main brake (i.e., foot brake) should be manipulated in order to offset the relative decrease of the entire braking force.
To reiterate, obtaining a sufficiently large engine brake force is important not only for the maneuverability and safety of the vehicle (engine brake force of approximately more than 60% of the rated output power is required), but also for taking better advantage of the advantages of the turbo compound engine. In connection with the above description, the present assignee has proposed a "Turbo Compound Engine" disclosed in Japanese Patent Application No. 61-228107 and U.S. Patent Application Ser. No. 091,161, filed Aug. 31, 1987.
In this proposal, as shown in FIG. 7, a power turbine which recovers the exhaust gas energy is provided in the exhaust gas passage b1, and another passage c which bypasses the turbine a is provided which is branched from b1 upstream of the turbine a, whereby to define a segment b2. An exhaust gas passage switching means e is provided so as to close the upstream side passage b2 when the exhaust brake is operating and driving power is transmitted to the turbine a from the crankshaft d.
The exhaust gas passage switching means e is comprised of a rotary valve which has two internal ports with large and small diameters in a manner such that the passage b1 and another passage c are connected to each other by the port of small diameter when the exhaust brake is functioning.
Upon exhaust-braking by the exhaust passage switching means, the exhaust gas line b1 upstream of the bypass line c is closed, and the upstream, side b2 of the power turbine a is connected with the bypass line c with the exhaust gas being throttled by the exhaust gas passage switching means e, so that the rotation of the crankshaft d is reversed by a gear train h and the rotative power is transmitted to the power turbine a, which is originally intended to recover the energy. In this manner, the power turbine a will perform negative work against the crankshaft, i.e., pumping work, since the exhaust gas downstream of the power turbine a comes through the bypass line c, as shown by the dotted arrow in FIG. 7. Therefore, during exhaustbraking, an engine brake force of considerable magnitude, that is to say, motor friction of the engine, pumping work (negative work), and the exhaust brake force, can be generated.
However, the exhaust brake may function, even when the engine is running above the rated revolution speed, i.e., when the engine is in an overrun condition, for example, in the case of driving down a slope under a light load-high revolution speed condition. In this case, there is a substantial possibility that the power turbine may also go into an overrun condition, which should be avoided. The present inventors have found that the overrun is a factor causing significant deterioration of rotation performance of the rotating portion of the power turbine as well as the bearing portion, and thus devised the present invention.